Attention to corrosion and corrosion mitigation have become increasingly important for economic and safety reasons. Based on estimates made in the mid 1990's, overall costs attributable to corrosion account for over $100 billion a year in the United States alone. These costs typically account for only the direct costs of corrosion and do not include the associated indirect costs, such as safety, plant downtime, loss of product, contamination and over-design.
Corrosion may be defined as the destructive effect of an environment on a metal or metal alloy. Nearly every metallic corrosion process involves the transfer of electronic charge in aqueous solution, and most corrosion reactions take place in the presence of water in either liquid or condensed vapor phases and also in high humidity. Corrosion is particularly a problem in marine environments experienced in places such as shipboard, aboard off-shore drilling rigs, and in coastal regions, among others, where seawater enhances corrosion reactions due to increased ion transport, pH effects, and elevated dissolved oxygen levels that in turn enhance levels of hydrogen ions. Corrosion reactions are further accelerated in marine environments by contaminants, such as chloride ions, present in seawater. Corrosion damage to equipment stored and used in marine environments is a tremendous problem, impacting maintenance costs, availability, repair, and reliability.
Equipment stored, e.g., onboard a ship or in coastal regions, is often stored in protective storage systems that have proved to be less than optimally effective. At best, such equipment is covered with waterproof tarpaulins, although often, especially for shipboard equipment, it is not covered properly and is directly exposed to a marine environment, which leads to rapid corrosion. Even when equipment is covered by waterproof tarpaulins, seawater still penetrates through and/or around the tarpaulins into the protected spaces where it collects and corrodes the underlying equipment. Also, conventional storage systems can be cumbersome to use and maintain, and are therefore often avoided. As a result, corrosion continues to be a significant and costly problem, requiring many hours of rust removal, painting, and repair that often lead to premature equipment replacement.
FIG. 1 shows a conventional waterproof cover 20 used to protect an object, such as metallic object 22 resting on a surface 24, from moisture, such as rain, sea spray, dew and the like. Cover 20 has an outer surface 26, an inner surface 28, and an area 30 defined by a peripheral edge 32. Cover 20 is shown covering object 22 in a typical manner, wherein a microenvironment is generally defined by the space enclosed by the cover. The microenvironment comprises a number of interior regions, such as regions 34, located between cover 20 and object 22.
Generally, conventional covers, such as cover 20, comprise at least one liquid-impermeable layer made of, e.g., a tightly-woven polymer fabric or a non-woven structure, such as a continuous film or other membrane. More complex conventional covers may include one or more additional layers that provide them with additional features, such as highly durable outer surfaces to withstand harsh environments and non-abrasive inner-surfaces to minimize mechanical damage to the object covered. Other conventional covers are made of vapor-permeable, porous materials, such as expanded polytetrafluoroethylene or the like.
The air in interior regions 34 generally never has a moisture content less than the moisture content of the ambient environment. If the moisture content of the ambient environment rises, the moisture content of regions 34 also rises due to the inflow of moisture (illustrated by arrow 36) through gaps between cover 20 and surface 24 at peripheral edges 32 of the cover. Eventually, the moisture content of the ambient environment 38 and regions 34 equalize. Once the additional moisture is in the microenvironment, it can become trapped, as illustrated by arrows 40. Moisture levels can quickly become elevated, and the air saturated. In such a case, condensation could occur on the object 22. Because the moisture content of interior regions 34 never falls below that of ambient environment 38, conventional covers are not very effective in high moisture environments, such as marine and high-humidity environments. Moreover, once moisture enters the microenvironment, it can take a long time to dissipate, if at all.